Inspired by a patch from Vivian, this adds a settable "WASTEGATE"

Inspired by a patch from Vivian, this adds a settable "WASTEGATE"
control to piston engines that allows external scripts to control the
turbo/supercharger boost programatically by setting this axis to
values in the range [0:1].  It also adds a "turbo-lag" attribute (a
time in seconds) to engines implementing turbocharger spooling delays.

This isn't terribly well tested, but doesn't seem to have broken
anything.